Objective Lens Design For Miniature Endoscope

ABSTRACT

An endoscope objective lens system design is disclosed, generally comprising a first lens group having overall negative refractive power, a second lens group having overall positive refractive power, and an aperture diaphragm disposed between the first and second lens groups wherein the first lens group includes a plano-concave lens with its concave surface facing the objective end of the endoscope.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of allowed U.S. patent application Ser.No. 11/735,198 filed Apr. 13, 2007, the content of which is incorporatedherein by reference.

FIELD OF THE INVENTION

This invention relates to an objective lens arrangement which can beutilized as an objective lens in an endoscope.

BACKGROUND OF THE INVENTION

Endoscopes generally include a series of lenses used to convey an imageof the scene to be viewed from a distal end of the endoscope to aproximal end, where the image may be observed by the user, captured byan image sensor, recorded by a video camera, and/or processed byelectronic means. (Throughout this application, the “distal” end of theendoscope will refer to the objective end, while the “proximal” end willrefer to the image end.) It is well known in the art that objective lenssystems in endoscopes often include two lens groups separated by anaperture diaphragm. As disclosed in U.S. Pat. No. 5,005,957 to Kanamoriet al. and U.S. Pat. No. 6,618,207 to Lei, the first lens group has anoverall negative refractive power while the second lens group has anoverall positive refractive power. This well-known optical design formof endoscope objective lens systems is referred to as reverse telephoto.

An endoscope objective lens arrangement 10 of the reverse telephoto typecan be seen in FIG. 1. Lens arrangement 10 has a first lens group 11having an overall negative refractive power nearest the distal end 14 ofthe endoscope, a second lens group 12 having an overall positiverefractive power nearer to the proximal end 18 of the endoscope, anaperture diaphragm 13 situated between said first lens group 11 and saidsecond lens group 12, and an image capture device 25. First lens group11 contains lens element 16 having a negative refractive power. Lenselement 16 is situated so that its concave surface 19 faces the aperturediaphragm 13 and the proximal end of the endoscope 18. First lens group11 also contains cover glass 15, which acts as a barrier between theoptical system and the environment external to the endoscope.

Endoscopes are intended to be used in minimally invasive surgicaltechniques, such as laparoscopy, hysteroscopy, and colonoscopy. As aresult, it is desirable to reduce the size of the endoscope as much aspossible in order to limit the stress on a surgical patient's bodytissues. The most important dimension of the endoscope for thesepurposes is its outside diameter, which is substantially limited by thediameter of the lenses inside the endoscope. Therefore, reducing thesize of the endoscope is best achieved by reducing the diameter of thelenses.

One advantage of the reverse telephoto configuration is that it allowsfor a relatively large field of view for an endoscope of small diameter.To obtain a large field of view, reverse telephoto designs generallyemploy one or more negative refractive power lens surfaces in the firstlens group. Negative refractive power lens surfaces having their concavesurfaces facing the aperture diaphragm usually provide most of thenegative power of the first lens group which has an overall negativepower. In many endoscopes, the first lens group is a single negativepower lens with its concave surface facing the aperture diaphragm.

As stated above, because of the intended application of endoscopes, itis desirable to reduce the diameter of the lenses as much as possible.Unfortunately, as a lens's diameter is reduced, its refractive powermust be increased in order to maintain the size of the lens's field ofview. As the diameter is reduced and the lens gets smaller, it becomesincreasingly difficult to manufacture the curved surface of the lens. Asa result, it is difficult and sometimes impossible to create a lens witha sufficiently small radius of curvature. Thus, as the lens diameterrequirements become more stringent and call for a smaller lens, itbecomes more difficult and sometimes impossible to manufacture the lenswith sufficient power due to the difficulty of creating a small radiusof curvature.

Endoscope designers are thus faced with a choice when utilizing theconventional approach to endoscope design. On one hand, the performanceof the optical system may be maintained at the expense of anysignificant reduction of the endoscope's size. On the other hand, theendoscope may be reduced in size at the expense of optical performance.It has been discovered that simply scaling a conventional endoscope lensconfiguration down to a smaller size is not possible. The overall sizeof the endoscope and the performance of the optical system are limitedby the manufacturability of the lenses.

U.S. Pat. No. 6,134,056 to Nakamuka is directed to designs of objectivelens systems for endoscopes that attempt to address these problems.Nakamuka discloses objective lens systems including a negative firstlens in the first lens group having a concave surface facing the distalend of the endoscope. However, each of these designs is limited by thefollowing condition:

2.0<f ₃/f<5.0

Where f₃ is the focal length of the third lens in the system and f isthe overall focal length of the system. Nakamuka imposes thisrestriction for the stated reasons that a value of f₃/f of less than 2.0would result in a larger-than-desired diameter of the third lens and avalue of f₃/f of more than 5 would introduce undesirable color shadinginto the image. However, it has been found that designing f₃/f to lessthan 2.0 does not require a larger diameter in the third lens while atthe same time minimizing color shading in the image.

Using conventional endoscope objective lens configurations has limitedthe creation of miniature endoscopes which would be of great value inthe field. What is desired, then, is an endoscope lens configurationdesign which minimizes the diameter of the lenses without sacrificingthe optical performance of the system to an extent that use of theendoscope becomes impractical or undesirable. It is also desirable thatthis design be simple and inexpensive to implement.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide anendoscope objective lens system design.

Another object of the present invention is to provide an endoscopeobjective lens system design which minimizes the diameters of the lensestherein and therefore minimizes the overall endoscope diameter.

Yet another object of the present invention is to provide an endoscopeobjective lens system design of minimal size without significantlysacrificing the optical performance of the endoscope.

Yet a further object of the present invention is to provide an endoscopeobjective lens system design which is simple and inexpensive tomanufacture and implement.

These and other objects are achieved in accordance with one embodimentof the present invention by providing an endoscope objective lensarrangement comprising a first lens group having a negative refractivepower and a lens in said first lens group that is plano-concave with itsconcave surface facing the distal end of the endoscope.

In a first embodiment of the present invention, an endoscope objectivelens arrangement having an overall focal length of f comprises: a firstlens group having a negative refractive power at a distal end of theendoscope; an aperture diaphragm disposed on a proximal side of thefirst lens group; a second lens group having a positive refractive powerdisposed on the proximal side of the aperture diaphragm; a first lensthat is plano-concave in the first lens group with its concave surfacefacing the distal end of the endoscope; a second lens that isplano-convex in the second lens group disposed immediately proximal tothe aperture stop; a third lens that is plano-convex in the second lensgroup disposed on the proximal side of the second plano-convex lens andhaving a focal length of f_(L); and wherein the following condition issatisfied: f_(L)/f<2.0.

Some embodiments may further include the features that substantially allof the negative refractive power of the first lens group is provided bythe first lens that is plano-concave, the first lens that isplano-concave has a radius of curvature of greater than or equal to 0.5millimeters, and any lens in the first or second lens group has adiameter of less than or equal to 1 millimeter. Some embodiments mayalso include a cover glass having negligible refractive power in thefirst lens group. Some embodiments may further include a fourth lens anda fifth lens adapted to form a doublet to correct for lateral coloraberration. In some embodiments the fourth and fifth lenses havesubstantially the same index of refraction but different dispersions.Some embodiments may also include electronic image processing means forcorrecting lateral color aberration and/or image distortion. Someembodiments may include a color correction filter or an infrared filter.Some other embodiments include a color-correcting filter and/or aninfrared blocking filter disposed within the objective lens arrangement.

In a second embodiment of the present invention, an endoscope objectivelens arrangement comprises: a first lens group having a negativerefractive power at a distal end of the endoscope; an aperture diaphragmdisposed on a proximal side of the first lens group; a second lens grouphaving a positive refractive power disposed on the proximal side of theaperture diaphragm; a first lens in the first lens group which isplano-concave with the concave surface facing the distal end of theendoscope; a second lens and a third lens disposed in either the firstlens group or the second lens group wherein the second and third lensesare adapted to form a doublet.

In some embodiments, the first lens provides substantially all of thenegative refractive power of the first lens group. In some embodiments,any lens in the first or second lens group has a diameter of less thanor equal to 1 millimeter. In some embodiments, the curved surface of thefirst lens has a radius of curvature of greater than or equal to 0.5millimeters. In some embodiments, the first lens group includes a coverglass having negligible refractive power. In some embodiments, thesecond lens and the third lens have substantially the same index ofrefraction but different dispersions. In some embodiments, the doubletis adapted to correct lateral color aberration. In other embodiments,lateral color aberration and/or image distortion is corrected by anelectronic image processing means. In some embodiments, the second lensgroup comprises a plano-convex positive lens element adjacent to theaperture stop with its convex surface facing the proximal end of theendoscope followed by a plano-convex element with its convex surfacefacing the distal end of the endoscope. In some embodiments, adistal-facing surface of the second lens and a proximal-facing surfaceof the third lens are planar. In some embodiments, a color-correctingfilter and/or an infrared filter is disposed within the objective lensarrangement.

In a third embodiment of the present invention an endoscope objectivelens arrangement comprises, in order from a distal end of an endoscope:a cover glass; a first lens that is plano-concave with its concavesurface facing the distal end of the endoscope; an aperture diaphragm; asecond lens that is plano-convex with its convex surface facing aproximal end of the endoscope; and a third lens and a fourth lensadapted to form a doublet.

In some embodiments, any lens in said objective lens arrangement has adiameter of less than or equal to 1 millimeter. In some embodiments, thecurved surface of the first lens has a radius of curvature of greaterthan or equal to 0.5 millimeters. In some embodiments, the third lensand the fourth lens have substantially the same index of refraction butdifferent dispersions. In some embodiments, the doublet is adapted tocorrect lateral color aberration. In some embodiments, a distal-facingsurface of the third lens and/or a proximal-facing surface of the secondlens are planar. In some embodiments, lateral color aberration and/orimage distortion is corrected by an electronic image processing means.In some embodiments, a color-correcting filter and/or an infrared filteris disposed at a selected location in the objective lens arrangement.

The invention and its particular advantages and features will becomemore apparent from the following detailed description considered withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings, the invention is demonstrated withoutrestricting its general inventive concept by means of the models shown.The illustrations are as follows:

FIG. 1 shows a conventional endoscope objective lens arrangement asfound in the prior art.

FIG. 2 shows a first embodiment of the present invention.

FIG. 3 shows a second embodiment of the present invention.

FIG. 4 shows a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 2, an endoscope objective lens arrangement 30having an overall focal length of f according to the present inventionis shown. Like the endoscope objective lens arrangement 10 shown in FIG.1, the objective lens arrangement 30 has a first lens group 31 nearestthe distal end 34 of the endoscope with an overall negative refractivepower, a second lens group 32 nearer to the proximal end 38 of theendoscope with an overall positive refractive power, an aperturediaphragm 33 between the first lens group 31 and the second lens group32, and an image capture device 45.

The first lens group 31 includes a plano-concave lens 36. Lens 36 ispositioned such that the curved surface 39 of lens 36 faces the distalend 34 of the endoscope. First lens group 31 also includes cover glass35 at the distal end 34 of the endoscope and serves to separate theoptical components of the endoscope from the external environment. Inthe embodiment shown in FIG. 2, lens 36 is responsible for all of thenegative refractive power of first lens group 31. It should be notedthat in other embodiments the first lens group could include otheroptical elements with either positive or negative refractive power.

The second lens group 32 includes plano-convex lens 46 immediatelyproximal to the aperture stop 33 and a second plano-convex lens 41 theconvex surface of which faces the distal end 34 of the endoscope andwhich has focal length f_(L). Lens 41 is designed such that f_(L)/f willbe less than 2.0. As stated above, this parameter allows for significantreduction of color shading in the image while not increasing thediameters of the lenses. The endoscope objective lens arrangement 30also contains a filter 40 for color correction and/or the reduction ofinfrared transmission. The filter 40 is shown in the second lens groupbut could also be disposed in the first lens group in other embodiments,or may be not be present at all in other embodiments.

FIG. 3 shows a second embodiment of the present invention. Like thefirst embodiment shown in FIG. 2, the endoscope objective lensarrangement 50 has a first lens group 51 at a distal end 54 with anoverall negative refractive power, a second lens group 52 at a proximalend 58 with an overall positive refractive power, an aperture diaphragm53 that separates the two lens groups, and an image capture device 65.

The first lens group 51 in lens arrangement 50 includes a plano-concavelens 56 having negative refractive power, the concave surface of whichfaces the distal end 54 of the endoscope. In this embodiment, the firstlens group 51 also includes a doublet 61 which consists of two lenses 62and 63. The doublet 61 has interface 64 where lenses 62 and 63 are incontact. The doublet 61 has a primary purpose of correcting lateralcolor aberration, and may correct other aberrations of the opticalsystem. The lenses 62 and 63 may be composed of two different materialshaving substantially the same index of refraction but differentdispersions. Lenses 62 and 63 may be constructed such that theirsurfaces opposite to interface 64 are either curved or planar. In someembodiments surface 64 may be concave to the distal end of the endoscope54, while in other embodiments it may be convex to the distal end 54.

The second lens group 52 has an overall positive refractive power nearerto the proximal end 58 of the endoscope. The second embodiment alsoincludes a cover glass 55 at the distal end 54 of the endoscope thatagain serves to separate the optical components of the endoscope fromthe external environment. The endoscope objective lens arrangement 50also contains a filter 60 for color correction and/or the reduction ofinfrared transmission. The filter 60 is shown in the second lens groupbut could also be disposed in the first lens group in other embodiments,or may not be present at all in other embodiments.

Referring now to FIG. 4, a third embodiment of the present invention isshown. The objective lens arrangement 70 has a first lens group 71 at adistal end 74 with an overall negative refractive power, a second lensgroup 72 at a proximal end 78 with an overall positive refractive power,an aperture diaphragm 73 that separates the two lens groups, and animage capture device 85.

The first lens group 71 in lens arrangement 70 includes a plano-concavelens 76 having negative refractive power, the concave surface of whichfaces the distal end 54 of the endoscope. In this embodiment, the secondlens group 72 also includes a doublet 81 which consists of two lenses 82and 83. The doublet 81 has interface 84 where lenses 82 and 83 are incontact. The doublet 81 has a primary purpose of correcting lateralcolor aberration, and may correct other aberrations of the opticalsystem. The lenses 82 and 83 may be composed of two different materialshaving substantially the same index of refraction but differentdispersions. Lenses 82 and 83 may be constructed such that theirsurfaces opposite to interface 84 are either curved or planar. In someembodiments surface 84 may be concave to the distal end of the endoscope74, while in other embodiments it may be convex to the distal end 74.

The third embodiment also includes a cover glass 75 at the distal end 74of the endoscope and again serves to separate the optical components ofthe endoscope from the external environment. The endoscope objectivelens arrangement 70 also contains a filter 80 for color correctionand/or the reduction of infrared transmission. The filter 80 is shown inthe first lens group but could also be disposed various locations withinlens arrangement 70 in other embodiments, or may not be present in otherembodiments.

The present invention is advantageously employed in endoscopes having asmall size. An objective lens system according to the present inventioncould be employed in an endoscope wherein the diameter of the largestlens in the system is 1 millimeter or less. In such a system, thepresent invention allows for the curved surface 39, 59, or 79 of theplano-concave lens 36, 56, or 76 in either the first lens group 31, 51,or 71 to have a radius of curvature of 0.5 millimeters or greater.

As described in U.S. Pat. No. 6,618,207 to Lei, the images generated byendoscopes are often distorted. Images generated by endoscopes can alsosuffer from lateral color aberration, wherein the red and blue colors ofan image become separated because of the different refractive tendenciesof red and blue light. This may also be the case in endoscopes thatutilize the present invention. Generally, the lateral color aberrationand/or distortion generated in an objective lens arrangement accordingto the present invention can be partly or fully corrected usingelectronic image processing means. Such electronic image processing isgenerally done by a computer or electronic processor integral to theendoscope system which receives the image from the endoscope, processesthe image to correct for distortion, and then displays the correctedimage on a monitor.

Although the invention has been described with reference to a particulararrangement of parts, features and the like, these are not intended toexhaust all possible arrangements or features, and indeed many othermodifications and variations will be ascertainable to those of skill inthe art. Accordingly, reference should be made primarily to theaccompanying claims, rather than the foregoing specification, todetermine the scope of the invention.

1. An endoscope objective lens arrangement, comprising, in order from adistal end of an endoscope: a cover glass; a first lens that isplano-concave, with its concave surface facing the distal end of theendoscope; an aperture diaphragm; a second lens that is plano-convex,with its convex surface facing a proximal end of the endoscope; a thirdlens and a fourth lens adapted to form a doublet.
 2. The endoscopeobjective lens arrangement according to claim 1 wherein any lens in saidobjective lens arrangement has a diameter of less than or equal to 1millimeter.
 3. The endoscope objective lens arrangement according toclaim 1 wherein the curved surface of said first lens has a radius ofcurvature of greater than or equal to 0.5 millimeters.
 4. The endoscopeobjective lens arrangement according to claim 1 wherein said third lensand said fourth lens have substantially the same index of refraction butdifferent dispersions.
 5. The endoscope objective lens arrangementaccording to claim 1 wherein said doublet is adapted to correct lateralcolor aberration.
 6. The endoscope objective lens arrangement accordingto claim 1 wherein a distal-facing surface of said third lens and/or aproximal-facing surface of said fourth lens are planar.
 7. The endoscopeobjective lens arrangement according to claim 1 wherein lateral coloraberration is corrected by an electronic image processing means.
 8. Theendoscope objective lens arrangement according to claim 1 wherein imagedistortion is corrected by an electronic image processing means.
 9. Theendoscope objective lens arrangement according to claim 1 wherein acolor-correcting filter is disposed at a selected location therein. 10.The endoscope objective lens arrangement according to claim 1 wherein aninfrared filter is disposed is disposed at a selected location therein.